The combustion of most fossil fuels such as coal and oil results in the production of sulfur dioxide by oxidation of sulfur impurities in the fuel. The resultant sulfur dioxide when emitted from the stack into the atmosphere is very objectionable and, in most instances, illegal. Consequently, considerable attention has been paid to the problem of removing the sulfur dioxide from the combustion gases, particularly in large steam generating installations, such as those of the utility industry, where large quantities of such fuels are burned.
It is now well known to decrease the emission of sulfur dioxide into the atmosphere by scrubbing the combustion gases, before they pass into the stack, with aqueous solutions and suspensions of reagents which combine with the sulfur dioxide. Several of these known wet methods involve the regeneration of the reagent and the production of sulfur-containing products having a market value. However, the operational costs of such methods are high and the sale of the resultant sulfur-containing products in the available market at a reasonable price is a problem. Other wet methods for the purpose involve the use of aqueous suspensions of finely divided, lime-bearing materials such as limestone and calcium hydroxide. The disposal of the resulting reaction sludge, which contains calcium sulfite and calcium sulfate formed by reaction, constitutes a major problem.
All the scrubbing methods have the disadvantages that the treated combustion gases, having lost buoyancy, must be moved through the scrubber by a fan, which consumes much power, and must be reheated, with a considerable input of thermal energy, before the pass into the stack. Another drawback of most scrubbing methods is that incrustations on the equipment must be cleaned therefrom periodically.
There are also known so-called dry methods of adsorbing or reacting with sulfur dioxide in combustion gases in which the gases are not cooled and a more easily disposable dry, spent material is obtained. In some of these methods, fine particles of lime-bearing materials such as limestone, dolomite, marl, burnt lime and calcium hydroxide are injected into the boiler. It is to be understood that, for convenience here and hereinafter, reference to "the boiler" or "a boiler" should be constructed as meaning the firebox used in heating such boiler and/or the flue immediately adjacent thereto. If ground or pulverized limestone is so injected, if must be initially decomposed to form CaO and CO.sub.2. This decomposition requires a considerable amount of heat as shown by the equilibrium: EQU CaCO.sub.3 .revreaction.CaO+CO.sub.2 -42.5 kcal
The CaO formed then reacts with sulfur dioxide, generating heat, according to the equation: EQU CaO+SO.sub.2 +1/2O.sub.2 =CaSO.sub.4 +119 7kcal
The overall reaction for the dry removal of SO.sub.2 is represented by: EQU CaCO.sub.3 +SO.sub.2 +1/2O.sub.2 =CaSO.sub.4 +CO.sub.2 +77.06 kcal
In practice, however, it has been found that only a minor part of the SO.sub.2 can be removed from the combustion gases by this method, even when an excess of lime-bearing material is used, that fouling and plugging occur in the boiler, as well as in the economizer and in the air preheater normally used in such systems, and that the performance of the electric precipitator, also usually employed for the removal of fly ash, is degraded due to the presence of a large amount of partially reacted dust. The low efficiency of the prior art, dry sulfur dioxide removal has two inherent reasons. Firstly, a shell of calcium sulfate forms by reaction around the lime particles through which SO.sub.2 cannot easily diffuse and the pores in the lime particles are clogged by the calcium sulfate. Consequently, not all of the CaO can be contacted by the SO.sub.2. In the second place, the effective time permitted for reaction of the lime-bearing material particles and SO.sub.2 is short, ranging from about 0.2 sec. to 2 sec. with usual boiler design. Matters are made even worse by the fact that the particles are usually not uniformly distributed in the combustion gases.
It has been found, as expected, that the reactivity of different lime-bearing materials varies. The least reactive is ground limestone. Calcined limestone (CaO) is more reactive and calcium hydroxide particles, which may have an average particle size of 0.2 micron, are still more reactive. Even when using the latter, however, only about 33% of the sulfur dioxide in the gases is removed in the use of known methods.
As indicated above, an excess of lime-bearing material is not able to increase substantially the efficiency of the processes just described and use of such an excess is not in general economically practical because of the increased cost of the lime-bearing material, the added dust problems in the boiler, economizer, air preheater, and electric precipitator, and because of the increased cost of disposing of the reacted material.